1. Field of the Invention
This invention relates to an improved arrangement for controlling the lifting and lowering of loads using hydraulics generally.
2. DESCRIPTION OF THE PRIOR ART
Previous systems employ multiple position spool valves to direct oil to and from the load lifting and lowering cylinder. Many systems include a maximum lowering speed control valve, usually attached to the cylinder. However, it is sometimes included with the main manual control valve. These lowering control systems usually have one or more of the following problems: instability; initial surge or lurch; poor lowering speed control vs. load; poor operator metering control; noisy operation; and high separate component cost.
Some systems incorporate pressure compensator spools within the main control valve. If these spools also act as a load check valve, initiation of a sudden damand for lift can cause a delay and then a pressure surge to appear in the lift cylinder. This is due to the high inertia of the relatively heavy compensator spool when it must suddenly move from a closed to an open position.
Some well known closed center load-sensing systems include a bleed-down type pilot circuit. These systems can have problems with the load drifting down slowly during demand for slow lift. In order to eliminate or minimize this problem, very critical land sequence tolerancing of two spool lands operating simultaneously is required.